1. Field of the Invention
The present invention relates to a hand-over processing method, and a hand-over processing system. More particularly, the present invention relates to a hand-over processing method, in which a subsequent (following) satellite takes over a communication service to continue a communication service without interruption in a low-Earth-orbiting (LEO) network, and a system for attaining it.
2. Description of the Related Art
In a conventional ground-based mobile communication system, a base station and a switching system are mainly fixed and a mobile unit (terminal) moves. In this mobile communication system, a hand-over process (communication succession process) that must be inevitably done when the mobile unit moves from a certain communication service area to a communication service area adjacent to the certain communication service area, is started (initiated) by the mobile unit when the mobile unit begins to move from the certain area to the adjacent area.
In the ground-based mobile communication system, a moving direction of the mobile unit and a starting point of the hand-over process are usually random. Thus, it is rare that a timing point when a certain mobile unit starts the hand-over process is equal to timing points when all other mobile units within the area from which the certain mobile,unit moves start the hand-over process on a particular base station, respectively. In other words, a possibility that a plurality of mobile units (mobile unit group) under management of a certain base station must simultaneously perform the hand-over processes on a particular base station is extremely low.
This kind of hand-over process technique is noted in, for example, Japanese Laid Open Patent Application (JP-A-Heisei, 10-243440). This noted technique has a system configuration in which the hand-over process is started by the mobile unit or the terminal to the base station.
By the way, in the LEO network, a satellite itself having functions as a base station and a switching system moves at a high speed in the skies of the mobile unit and the fixed terminal along a predetermined orbit.
Thus, if an ROFA (Region Oriented Frequency Access) method is applied as a beam scanning method to the ground mobile units and terminals from the satellite, a certain satellite providing a communication service to mobile units and terminals within an area, after a certain time, flies away from the skies of the area, and after an elapse of a predetermined time, can again start providing the communication service to the mobile units and the terminals within the area. In this case, it is inevitably necessary to simultaneously perform the hand-over processes with regard to the mobile units and the terminals within the area, on a following satellite which orbits around the Earth in the same orbit as the orbit of the certain satellite and directly follows the certain satellite and flies over the skies of the area, for each timing point when the certain satellite can not provide the communication service to the area, namely, periodically. Then, the simultaneous hand-over processes must be done periodically and repeatedly within a short time, as mentioned above.
Thus, if the technique noted in the above mentioned gazette is merely applied to such an LEO network, the simultaneous hand-over processes must be done periodically for each short time.
Thus, an amount of control information on a radio link between the mobile units or the terminals and the satellite can not be avoided from being suddenly increased for each relatively short time.
Accordingly, an amount of information that can be transmitted on the radio link is expended on the amount of the control information, which not only drops a usage efficiency of the radio link but also increases a process delay caused by the sudden increase of the amount of the control information. This results in the reduction of the communication service.
Japanese Laid Open Patent Application (JP-A-Heisei, 7-75156) discloses a conventional technique as a cellular communication system. In this system, a cellular communication network includes a satellite throwing a cell on the earth. The satellite and the cell move with respect to the earth. A call is set up to a subscriber unit, the date indicating the position of the subscriber unit is transmitted to a exchange unit of the cellular communication network. The exchange unit generates a schedule in which an advance notice of a time when the subscriber unit passes over a boundary of the cell is given. While the call is in progress, the subscriber unit judges whether or not a cross of the pre-noticed boundary is generated. The call is handed-off to a predetermined cell as far as an appropriate signal exists in a planed cell and as far as a network capacity exists in the predetermined cell.
Japanese Laid Open Patent Application (JPA-Heisei, 10-32532) discloses a conventional technique as a method of performing a hand-off of communication between a first satellite system 500 and an alternate satellite system. In this method, a case that a hand-off condition exits with respect to the first satellite system is determined. An alternate satellite of which a coverage area is positioned such that the alternate satellite can provide an alternate communication link is determined. After that, the hand-off between the first satellite system and an alternate satellite system is performed. If the condition of the hand-off is eased, a re-hand-off to the first satellite system is performed, and the normal operation is started again.